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March 25, 2010, 14:37 
Possible bug (limitation?) in setValues

#1 
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Alberto Passalacqua
Join Date: Mar 2009
Location: Ames, Iowa, United States
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Hello,
in trying to use setValues to exclude cells were the momentum equation is not defined (rho ~ 0) from the matrix, I found a potential bug/limitation (if I didn't do anything wrong in my code ). Let's assume the momentum equation is in the compressible form, and I want to exclude cells were rho < SMALL. I loop to identify those cells: Code:
labelHashSet doNotSolveMomentum; forAll(rho, cellI) { if (rho[cellI] <= 1.0e6) { doNotSolveMomentum.insert(cellI); } } Code:
labelList noMomentumCellLabels = doNotSolveMomentum.toc(); vectorField fixedU (noMomentumCellLabels.size()); forAll(fixedU, cellI) { fixedU[cellI] = vector(0.0, 0.0, 0.0); } UEqn.setValues(noMomentumCellLabels, fixedU); Code:
myA = UEqn.A(); forAll(rho,i) { if (rho[i] < 1.0e6) { Info << "rho = " << rho[i] << " A = " << myA[i] << endl; } } Is this the expected behaviour of setValues? Shouldn't A be set to 1 in those cells, so that the equation actually degenerates in U = myValue? Best,
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Alberto Passalacqua GeekoCFD  A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats. OpenQBMM  An opensource implementation of quadraturebased moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using. 

March 26, 2010, 04:16 

#2 
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setValues currently does not reset the diagonal, it uses the value the matrix provides to set the value.
H 

March 26, 2010, 05:29 

#3 
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Hrvoje Jasak
Join Date: Mar 2009
Location: London, England
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Hi Alberto,
You cannot just go in and change the diagonal value to 1 because the order of magnitude of other terms in the matrix is unknown: other diagonals can be anything betwees, say 1e10 and 1e10, depending what you're solving. If you give the diagonal of a wrong order of magnitude, it will change matrix normalization and that makes the solver behave in unexpected ways. I had this trouble with the FEM solvers a long time ago and the solution was to leave the diagonal as is. How come you have a zero diagonal in the first place? Can you estimate the size of the diagonal coefficient, eg. cell volume/delta t? Hope this helps, Hrv
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March 26, 2010, 12:04 

#4  
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Alberto Passalacqua
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Location: Ames, Iowa, United States
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Thanks to both of you for your replies.
Quote:
At the moment I am using a flag to set up the fvMatrix, doing something like Code:
fvVectorMatrix UEqn ( solveMomentum* ( fvm::ddt(...). + ... ) + doNotSolveMomentum* ( fvm::Sp(Uunits,U) ) ) Best,
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Alberto Passalacqua GeekoCFD  A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats. OpenQBMM  An opensource implementation of quadraturebased moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using. 

March 26, 2010, 20:45 

#5 
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Alberto Passalacqua
Join Date: Mar 2009
Location: Ames, Iowa, United States
Posts: 1,907
Rep Power: 27 
I forgot to answer to one point: in my case both the cell volume and the time step are usually constant (static mesh, fixed dt), so their ratio is known.
Best,
__________________
Alberto Passalacqua GeekoCFD  A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats. OpenQBMM  An opensource implementation of quadraturebased moment methods. To obtain more accurate answers, please specify the version of OpenFOAM you are using. 

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